Electric signal synchronising apparatus



June 2 5," 1957" K. c. JOHNSON 2,797,378

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7 ELECTRIC SIGNAL SYNCHRONISING APPARATU S I Filed March 24, 1955 sSheets-Sheet s FIG 4 [NYE/V T R June 25, 1957 K. c. JOHNSON ELECTRICSIGNAL. SYNCHRONISING APPARATIJS Filed March 24, 1953 5 Sheets-Sl ieet 4I/v VE/V TOR Mud-W 3 g 1 m9 W% 8:

QM Qw 5 @w 3 MG 2 h .W n 3 Q 8 H 5 mm B Q M? Q. a H ww June 25, 1957 K.c. JOHNSON ELECTRIC SIGNAL SYNCHRONISING APPARATUS Filed March 24, 19535 Sheets-Sheet 5 w m8 8N$ A AA A A v I l/VIEA/ T R m6 mmw ATsa we aELECTRIC SIGNAL SYNCHRONISING APPARATUS Kenneth C. Johnson, Manchester,England, assignor to National Research Development Corporation, London,England, a British corporation Application March 24, 1953, Serial No.344,366

Claims priority, application Great Britain March 26, 1952 19 Claims.(Cl. 318-302) This invention relates to apparatus for synchronisingelectric signals having pulsed waveforms and in particular to apparatusfor synchronising each of the pulses of a first series of pulses whoseperiodicity is controllable with related pulses in a second series ofcylically repeated control pulses. A particular but by no meansexclusive application of the invention is to synchronise the pulses of acontinuous train of slave pulses to each nth pulse of a cyclicallyrepeated train of pulses.

Such apparatus has useful application, inter alia, to binary digitalcomputers employing both a cathode-raytube data store (as described inBritish patent specification No. 645,691 and in Proc. I. E. E., partIII, March 1949, pages 81 to 100 and part II, February 1951, pages 13 to28) and also employing a magnetic data store in the form of a rotatingdrum (as described in copending patent application Serial No. 146,446,filed February 27, 1950, by Frederic C. Williams, Patent No. 2,652,554and in Pros I. E. E., part II, pages 29 to 34 and part II, April 1952,pages 94 to 106).

In such last mentioned particular application it is convenient tosynchronise the speed of the rotating drum with the set rhythmicoperation, i. e. the line scanning or beat period, of thecathode-ray-tube store which is continuously determined by a master orclock oscillator. In apparatus hitherto used for efiecting this, thereis generated a signal of square waveform in synchronism with each nthpulse of the train of digit pulses occurring during each line scanperiod of the cathode-ray-tube. This square wave signal has a periodtime equal to the time interval between successive nth pulses, i. e. thebeat interval of the machine, and is symmetrical about the wave axis,that is to say the duration of its positive-going period is equal to theduration of its negative-going period. From the rotating drum there isderived, by suitable electromagnetic pick-up means, a train of slavepulses having a frequency which is proportional to the speed of rotationof the drum. The phase of these slave pulse signals is then compared ina phase discriminator with the phase of the aforesaid square wave and anoutput voltage from the phase discriminator, indicative of phasedivergence, is applied to servo apparatus for controlling the speed ofrotation of the drum whereby the drum is constrained to rotate insynchronism with the remainder of the computing machine. This phasediscriminator is designed to give Zero output when the centre point ofeach slave pulse coincides in timing with that edge (hereinafter calledthe reference edge) of a cycle of the square wave signal which occurs insynchronism with the corresponding nth pulse of the train of digitpulses and to supply an output of one polarity or the other polaritywhenever the slave pulses slip away from the reference edge due to phaselag or phase advance as the case may be. The resultant output ofappropriate polarity delivered by the phase discriminator thus socontrols the speed of rotation of the drum as to adjust both therepetition frequency and the timing of the slave pulses relative to suchreference edges of the square waveform in a sense appropriate nitedStates Patent to arrest the slip and restore phase coincidence andthereby to maintain the drum rotating at the correct speed and at thecorrect instantaneous angular positional relationship with respect tothe operating rhythm of the machine whereby signals derived fromelectromagnet pickup means associated with other recording tracks on thedrum occur in correct time relationship with the similar signals in theremainder of the machine.

Such an arrangement works satisfactorily when the slip is slow butwhenever the slip is rapid, i. e. whenever the frequency of the slavepulses is widely different from the frequency of the control pulses andthe derived master square wave signal, there is necessarily applied tothe output smoothing circuit of the phase discriminator, voltages whichalternate rapidly between positive and negative polarity as the slavepulses slip past the respective half cycles of the master squarewaveform and as this latter waveform is symmetrical about the wave axis,the smoothed output voltage from the phase discriminator over severalslip cycles is zero and no control voltage is derived. Such a failure ofcontrol, is of course, well-known where the inputs to a phasediscriminator differ widely in frequency.

The object of the present invention is to provide apparatus of the typestated which is effective, despite wide differences between therepetition frequencies of the master and slave pulse waveforms.

In accordance with the broadest aspect of the invention, apparatus forsynchronising electric signals having pulsed waveforms comprises anelectric square waveform generator which is adapted to be triggered byeach of a series of control pulses to generate a master signal of squarewaveform having a period time equal to the time interval betweensuccessive master control pulses, a phase discriminator for comparingthe phases of said master square wave signal and of input slave pulseswhich are to be synchronised therewith and for deriving an outputvoltage which is of one or the other polarity according as said slavepulses are predominantly in phase with the positive or negative halfcycles, as the case may be, of the master square waveform signal, servomeans adapted to be actuated by the output of said phase discriminatorand when so actuated to adjust the periodicity of said slave pulses in asense dependent upon the polarity of said output and frequencydiscriminating means which is adapted to be actuated by the phasediscriminator output when the frequencies of said master square wavesignal and said slave pulse signals are unequal for rendering the squarewaveform of said master signal asymmetrical with respect to the waveaxis whereby the mean value of said phase-discriminator output over aplurality of slip cycles is not zero but is of a polarity appropriate tocorrect the frequency of said slave pulses.

In a particular embodiment of the invention, applied to thesynchronisation of each slave pulse with each nth pulse of a cyclicallyrepeated train of pulses, said master square-waveform generatorcomprises a two stable state trigger circuit, e. g. of the so-calledEccles-Iordan type, which is adapted to be triggered from a first stablestate to its second stable state by each of said nth pulses and to berestored from such second stable state to its first stable state byeither the ensuing (n+m)th pulse or the (n-m)th pulse as determined bythe polarity of the output voltage from said phase discriminator.

In the above and hereinafter the frequency when used with respect to theslave pulse means the slave pulse repetition frequency, while the termpredominantly in phase with a half cycle of the master square wave isintended to mean with more than half the length of any one the slavepulse being considered is in phase with a part of that half cycle.

In order that the nature of the invention may be more readily understoodone embodiment thereof as applied to an electronic digital computingmachine will now be described with reference to the accompanyingdrawings in which:

Fig. 1 is a block schematic diagram illustrating certain elements of anelectronic digital computing machine and an associated magnetic drumstore provided with a drum synchronising arrangement according to thepresent invention.

Fig. 2 illustrates in somewhat greater detail certain of the basicwaveform generating means of the computing machine which are concernedwith the drum synchonising operations. V V

Figs. 3 and 4 each comprise a series of explanatory waveform diagrams. I

Fig. 5 isa detailed circuit diagram showing the parti'cular arrangementsof the master square-wave generator, the phase discriminator, the switchvalve and visual indicator and the coarse speed servo control while Fig.6 is a detailed circuit diagram showing the arrangement of the shapingor stabilising valve and the final output power amplifier which suppliesthe brake control current. I 7

Referring first to Fig. 1 MS indicates the main data store of thecomputing machine comprising a device of the cathode ray tube type asdescribed inthe aforesaid references. As both the manner of operationand the construction of such devices are now well-known in the art andas adequate information thereon is obtainable from the references quotedit will not be described further. Such a store forms part of digitalcomputing machines operating in the series mode with pulse trainsrepresenting both numbers and instructions and this again has beendescribed in detail in the quoted'references.

Such machines operate in a series of major cycles or bars during each ofwhich one computation step out of a programme of computation steps isperformed. Each major cycle or bar comprises a plurality of minor orsubcycles known as beats during which the required next instruction isfirst selected and transferred to a control unit CL where it, in turncontrols the selection of and the setting up of the appropriate transferroute for (together with any accompanying arithmetical operation) arequired number which is also held within the main data store. Theoperating rhythm of the machine is determined by a series of electricwaveforms which are generated within the unit WGU which will bedescribed in greater detail later. Both numbers and instructions arerepresented within the machine, in their dynamic form, as electric pulsesignal trains in which the presence of a pulse at a predetermined timeinstant within any beat period indicates the binary value 1 and theabsence of a pulse in the same interval indicates thebinary value 0, theposition of the interval within the beat period being indicative of thedenominational or power value of the binary digit itself.

Such machines are desirably provided with a large storage capacity forrecording data items and in practice cathode ray tube type stores becomeunnecessarily bulky and complex if used for retaining all of therequired data items required in a long programme. Such machines aretherefore frequently provided with a subsidiary store in the form of amagnetic drum MD having a magnetic recording layer around itscircumferential surface to provide a plurality of endless data storagetracks DT in each of which a plurality of magnetic storage patterns eachrepresentative of one number or instruction word may be recorded. Thetotal number of separate word 10- cations in any one track is made equalto an integral number, usually two, complete fillings for a cathode raystorage tube. Each track DT is provided with its own read/write head DHby which input electric signals can be caused to make the requiredmagnetic recording and from which output signals representative of thepreviously recorded signals, can later be derived. Arrangements 4 areprovided for selecting any required read/write head and hence any onetrack for use and then transferring signals between such track and themain data store MS whereby the contents of a cathode ray storage tubecan be transferred en bloc to a magnetic storage track or vice versa.The selection of the required read/write head and its associated datatrack DT is effected by means of a track selecting relay tree circuit RTwhich is controlled in its operation by electric signals derived fromthe control unit CL of the computing machine in response to the obeyingof a particular instruction in the machine. The signals transferredbetween the magnetic drum MD and the main data store MS pass throughread and write circuits WRC whose purpose is to convert the normaldynamic signal pulse trains circulating within the machine to a formsuitable for energising the heads DH to effect magnetic recording and,conversely, for converting the pick-up output signals derived from suchheads back into pulse train signals of the type circulating within themachine. As such apparatus forms no part of the present invention andhas already been described in the quoted literature it will not befurther referred to.

For satisfactory operation during transfer of signals in eitherdirection between the stores it will be apparent that both the speed ofrotation of the drum MD and its angular position at any instant mustbear a predetermined relationship to the beat period of the computingmachine if the signals recorded in each of the word storage locations ofa track around the magnetic drum by signal pulse trains delivered fromthe store MS are subsequently to be reproduced and converted back intosignal pulse train form with a timing, relative to the machine beatperiod, which renders them suitable for direct use within the machine.

This required synchronism is obtained byarranging for the magnetic drumto be driven continuously by an electric motor DM, conveniently a3-phase A. C. motor, energised over leads from a suitable supply sourceand with such motor arranged so that it inherently tends to drive thedrum at a rather faster speed than that which iscorrect to maintainsynchronism with the associated computing machine. Directly coupled tothe drum MD is an electromagnetic braking means BM, conveniently in theform of a single-phase A. C. induction motor, which is supplied with adirect current of variable value whereby it operates as an eddy-currentbrake to maintain,by appropriate adjustment of the applied brakingcurrent, theresultant rotation of the drum at both the correct angularspeed and with the correct angular relationship of any given point onthe drum to the operating rhythm of the computing machine.

The drum MD isprovided with a further, slave pulse, recording track STin which are initially recorded a series of short pulse signals, oneforeach of the word storage locations in the other date storage tracksDT. Each slave pulse therefore corresponds to one line scanning motionof the main cathode ray tube data store MS. Thus, if each cathode raytube of the main data store MS records 64 20-digit number or instructionwords and if each magnetic recording track has a capacity of storing twocomplete fillings for a cathode ray tube store then there will be 128separate word storage locations in each data storage track'DT and 128individual slave pulse signals recorded at equally spaced intervalsaround the slave pulse track ST. This slave pulse track 'ST has anassociated read or pick-up head SRH by which the series ofslave pulsesignals may be continuously derived and made available on lead 101.These signals, after suitable amplification in amplifier SPA, areapplied by way of lead 102to-one input terminal 103 of a phasediscriminator PDC the other-input terminal 104 of which is suppliedwitha master-square'wavefor'm by way of lead from a master square Wavegenerator MSG. This master square wave generator MSG 'is controlled, ina manner described indetail later by certain timing waveforms of thecomputing machine which are made available over leads 106, 107 and 108from the waveform generator unit WGU and also by output voltagessupplied by way of leads 109 and 110 from a coarse servo control valvecircuit CSC and a switch valve circuit SVC respectively.

The output from the discriminator over lead 111 is zero provided theslave pulse signals obtained from the slave pulse track ST aremaintained in correct alignment with the so-called reference edge of themaster square-wave delivered from the master square-wave generator MSG.Such reference edge is the point of abrupt change from negative topositive value of the master square-wave which occurs in synchronismwith each nth or control pulse. If the slave pulses occur in advance ofsuch reference edge the phase discriminator output will be of negativepolarity whereas if such slave pulses are lagging on such reference edgethe phase discriminator output will be of positive polarity. The outputfrom the phase discriminator PDC is applied over lead 111 to a shapingvalve SV whose output is fed by way of lead 112 to a power amplifier PAwhich in turn supplies the current for energising the electromagneticbraking means BM by way of leads 113. The output from the phasediscriminator PDC is also fed by means of lead 114 to control the switchvalve circuit SVC while the latter operates a visual Fast/Slow indicatorIND over lead 115. A further output from the phase discriminator PDCcontrols the coarse servo valve circuit CSC over lead 116.

Referring now to Fig. 2 the Waveform generating arrangements of unit WGUof the computing machine comprise a stable-frequency master or clockoscillator CPG operating at 100 kc./s. and providing a square pulse orClock waveform of microseconds periodicity as shown in Fig. 3a. Thisoscillator serves to control the basic operating rhythm of the machine.The output Waveform from this clock oscillator CPG is applied to a firstpulse dividing circuit DV1 which counts down by a factor of 4 to providean output waveform DV1 as shown in Fig. 3b consisting of a square pulsein synchronism with every fourth Clock pulse. The output from thisdivider circuit DV1 is, in turn, applied to a second divider circuit DV2which counts down by a factor of 6 to provide an output waveform DVZ asshown in Fig. 3c, consisting of a square pulse in synchronism with every24th Clock pulse. This output from divider circuit DV2 defines the beatperiod of the machine rhythm as comprising a total of twenty-four Clockintervals. Twenty of these clock intervals are used for signalling therespective binary digits of a -digit binary number and the remainingfour clock intervals are needed for accommodating the fly-back motion ofthe scanning beam in the cathode ray tube store. This fly-back period isdefined by the negative-going pulse period of the blackout or B0waveform, Fig. 3a. This waveform is generated in a trigger circuit BOPGwhich is set into one state by each output pulse from the dividercircuit DV2 and is reset back into its original state by the nextfollowing output pulse from the divider circuit DV1 whereby the blackoutpulse period embraces the first four clock intervals of each beat andthe remaining twenty Clock intervals are available for digit signallingpurposes.

The dynamic form of the number and instruction signals used within themachine take the form shown in Fig. 3e and comprise a train of pulses,one in each of those of the 20 clock intervals assigned to digitsignalling where it is required to represent the binary digit 1. Theabsence of a pulse in any digit interval indicates binary value 0. Each1 representing or Dash pulse is negative-going and persists for thefirst 6-microseconds of the IO-microseconds clock or digit interval.These 1 representing pulses are derived, by suitable selection, from acontinuous train of Dash pulses, shown in Fig. 3 and generated in asquare pulse generator circuit DSPG which is repetivitely triggered bythe Clock waveform. The position of a pulse in the group of 20successive digit 6 intervals of each beat is indicative of the binarypower value or denomination which it represents. Thus the first digitinterval (the 5th clock pulse) in each beat may be indicative of thebinary value 2 and is known as the p0 interval, the second digitinterval then representing binary value 2 and being known as the p1interval and For the purpose of selectively examining any one of the 24Clock or digit intervals of each beat period there is provided a seriesof repetitive pulsed waveforms each on separate leads and eachcomprising one pulse in each beat period coincident respectively witheach of the different digit or clock intervals. These pulses, known asp-Pulses, are generated in a unit PPG which comprises a series of 24combined trigger-gate circuits P0, P1, P2 P23. Each trigger circuitcontrols its associated gate to open the latter when the trigger circuitis in one (triggered) state and to close such gate when the triggercircuit is in the opposite (reset) state. All the gates are supplied inparallel with the continuous train of Dash pulses, Fig. 3 f, and thetrigger circuits are interconnected as a counterchain whereby eachtrigger circuit is triggered by the passage of a Dash pulse through thegate of the previous trigger circuit of the chain and whereby suchtrigger circuit is, in turn, reset to its normal or gateclosed conditionupon triggering of the next following trigger circuit of the chain. Thefirst trigger circuit P20 of the group is triggered by the output fromthe divider circuit DV2 whereby it allows the dash pulse of the nextclock interval, p20, to pass therethrough whereupon that trigger circuitis reset and the next circuit P21 is triggered to allow the nextfollowing Dash pulse in clock interval 221 to pass therethrough and soon, thus to provide the series of so-called p-Pulse waveforms of whichthose coincident with the digit intervals p23, p0, p7 and p15 areillustrated in Figs. 3g, 3h, 3i and 3 respectively.

The slave pulse signals recorded in the track ST of the magnetic drum MDcomprise a series of pulses coincident in timing with those instantswhen the storage positions in the other data tracks DT corresponding tothe p23 Clock interval are passing the read/write heads DH for suchother tracks whereby a transient output signal output as shown in Fig.3l is obtained from the read head SRH once immediately before thecorresponding twenty digit recording positions for a data word signalare scanned by the read/Write heads DH of the storage tracks DT. It willbe observed that the positive transient pulse has its peak coincident intiming with the leading edge of the p23 Pulse when exact synchronismoccurs. The master square-wave generator MSG is triggered by suchleading edge of each p23 Pulse of the p-Pulse waveforms whereby itreverses its state to provide the reference edge at that instant of themachine operating rhythm as indicated at x in Fig. 3m and thesynchronisation problem is therefore to maintain the slave pulse signalsre ceived from the track ST of the magnetic drum in exact register withthe abrupt change or reference edge of the master square-waveform. Thisis effected as will be explained in detail later, by causing theaforesaid reference edge of the master square-wave effectively to dividethe positive transient portion of the slave pulse into two parts andthen to compare them for equality whereby no re sultant output signal isobtained when the parts are equal. Whenever a slave pulse is out ofexact register with the reference edge of the master square-Wave it willbe predominantly in phase with either the negative-going portion of themaster square-wave which precedes the reference edge or alternativelywith the positive-going portion which succeeds such reference edge andthe output signal will accordingly be of negative or positive value, theamplitude of signal varying according to the degree of divergence withexact synchronism until the degree of slip is such that the whole of theslave pulse occurs during the positivcor negative-going part of themaster square-wave.

If the slip is large, thatis to say, the drum speed is widely in errorthen the slave pulses will rapidly passthrough the positive and negativeportionsofthe master square-wave in succession and, over a few cyclesofoperation, the mean output from thephasediscriminator will again be zeroif, as hitherto, the master square-wave is made symmetrical about itsaxis with the positive-going portions equal in length to thenegative-going portions. When this occurs all control willfaila In thepresent invention the output from the discriminator is arrangedtocontrol-the-formof the master square wave so that it is asymmetrical,that is to say either its negativeor its positive-going portion'is ofgreater duration than the opposite negativeor-positive-going portion asthe case may be-whereby the mean output from the phase discriminatorwill not be zero when the slip is large but will be of. a polarity suchthat the tendency is tocorrect the error. Thus, if the slave pulsesoccur at a lower frequency than the reference edges due to the drumspeed being too low, the positive portion of the master square-wavefollowing the reference edge region willbe extended so that, in the.time interval before the slave pulses pass through the next followingreference edge, the number of instances when the slave pulse occursduring the positive portion of the master square-wave will outnumber theinstances when the slave pulses occur during the negative-portion of thesame master squarewave cycle thereby giving a mean output from thediscriminator which is positive and which reduces the'applied brakingeffects. Conversely, if the slave pulses occur at a greater frequencythan the reference edges of the master square-wave, due to the drumspeed being too high, the preceding negative portion of the mastersquare-wave will be extended togive, in similar manner, a mean outputfrom the discriminator which is negative and which causes anincrease ofthe braking force. The aforesaid slave pulse dividing action willfinally take charge when the speed and angular-position of the drum isnearly correct toobtain and maintain accurate synchronism. Thesearrangements are effective between about A correct speed and the exactlycorrect speed of operation of the drum relative to the machine rhythm.For ensuring satisfactory operation in the still lower speed range whenthe drum speed is building up from rest a coarse servo-control isprovided which temporarily overrides the above described arrangementsand causes locking of the master square-wave generator in the conditionwhere it provides a positive-going portion subsequent to each referenceedge which is longer thanthe related negative going por- Theservo-control arrangement will now be described in detail with referenceto Figs. 5 and 6. Referring to Fig. 5, the master square-wave generatorMSC comprises the valves V1, V2 (conveniently a double triode valve) anddiodes D1, D2 and D3, while the phase discriminator PDC comprises thevalves V4, V5 (also a double triode valve) and valve V6 withdiodes D6,D7, D8 and D9. Valves V7 and V8 constitute the switch valve circuit SVCand valve V3 with diodes D 1, D5 constitutes the coarse servo valvecircuit CSC.

In the master square-wave generator MSG, valves V1 and V2 are arrangedas a two-stable-state trigger circuit by conventional cross-connectionof their anodes and control grids by way of resistors R1, R2 withshunting condensers C1, C2 the anode load of valve V1 being constitutedby resistor R3 and that of valve V2 by resistor R4, both connected tosource of positive potential +300 v. Resistors R7, R8 constitute gridleak resistances for valves V1 and V2 respectively to a source ofnegative potential l50 v. The cathode of each valve is earthed. The,control grid of valve V1 is. supplied by way of input lead 106 and diodeD1 with the pZS-Pulsewaveform,

Fig. 3g and Fig. 4a, whereby the triggercircuit is trig gered by theleading edge of such p23-pulse to-the condi-- tion -wherevalve-Vl iscut-offandvalve V2 is conductivewith its grid at or about earthpotential. This provides the reference edge x, Fig. 3m and'Figs. 4d and4e.

The grid ofvalve V2" is similarly-supplied by way of lead 107 and diodeD2 with. the plfi lulsewaveform, Fig. 3i and Fig; 46 whereby, unlessother conditions, to be described later, arise, the trigger circuit isreset by the negative-ed'geof the-p15 Pulse=waveform whereby valve V2 iscut-offend valveVL is rendered conductive. The output from the triggercircuit under these conditions isindicated inFig. 3m (full and chain-dotlines) and in Fig.- 4d, in which the return or intermediate edge y ofthe master square-wave occurs. at the instant of the p15 pulse in eachbeat.

the p7-Pulse waveform, Fig. 3i and Fig. 4c. D3 is controlled by theswitch valve circuit SVC' and if rendered conductive thereby, thep7-Pulse waveform is then operative to reset the trigger circuit ofvalves V1 and V2 earlier ineach cycle of the master squarewaveto producean output waveform as shown in Fig. 3m (full and dot'lines) and'in'Fig.4e where the intermediate edge y occurs at the instant of the )7 pulsein each beat.

In the phase'discriminator PDC valves V4 and V5 have their respectiveanodes connected to opposite end terminals of the'primary winding of acentre-tapped transformer T which centre tap is connected to a source ofpositive .potential- +300-v. The control grid of valve V5 is takendirectly to a source of positive potential v. whereas the control grid'of valve V4 is connected by way of a resistor R11 to the junction pointof apotentiometer network of resistors F9 and R10. The opposite end ofresistor R9 is connected by way of the input terminal 104- and' lead 105to the anode to a source of negative potential 150 v.

up the operation of the circuit.- The junction point between resistorsR9, R10 leading to the control grid of valve V4, is also connected tothe anode of a diode D6 whose cathode is taken to a source of positivepotential v. and to the cathode of a diode D7 whose anode is taken-to asource of positive potential +85 v. These two diodes serve to limit thevoltage swing at the grid of valve V4 within the range of +115 v. to +85v. When the trigger circuit of valves V1 and V2 is in its triggeredcondition, i; c. with valve V1 cut olf and valve V2 conductive togivethe positivegoing portion of the ma er square-wave, the anode ofvalve V1is higher than +115-v. whereby the control grid of valve V4-is driven to+115 v. When however the trigger circuit of valves V1 and V2 is resetandvalve V1 is conductive then its anode is lower than +85 v. and thecontrol gridof valve V4 is driven to The two cathodes of valves V4,VSare. interconnected and joined'by way of a load resistor R12 to theanode of a pentode valve Vfi-whose screen grid is supplied with itsoperating potential byway of resistor R15 con-- head SRH-whichisassociated with the-slave pulse track The control gridof' valve V2 isalso connected fed byway of diode D2- and condenser C3 to lead-l08 whichis suppliedwith The diode.

R16 and condenser C is joined by way of leak resistance R17 to a sourceof negative potential 12 v. whereby valve V6 is normally cut-off and isrendered conductive only during the positive transient peaks of theoutput waveform from the read head SRH which is shown in Fig. 31.

When valve V6 is cut-01f, valves V4 and V5 are likewise inoperative asthere is no available supply of space current thereto. When valve V6 isturned on by the positive transient of the Slave pulse signal, anodecurrent will flow in Whichever one of the valves V4 or V5 is at thattime rendered conductive. If the trigger circuit of valves V1, V2 is inits triggered condition with the anode of valve V1 high in potential,then valve V4 alone of the pair V4, V5 will conduct and current willflow through one half of the primary winding of transformer T. If,however, the trigger circuit V1, V2 is reversed or in its reset statethen the control grid of valve V5 will be more positive than the controlgrid of valve V4 and current will flow only through valve V5 and theopposite half of the primary winding of transformer T.

The secondary winding of transformer T has one end connected to earthand its opposite end connected in parallel, by way of condensers C8 andC9 to diodes D8, D9, the latter being reversed in their polarity wherebythe cathode of diode D8 is connected to the condenser C8 and the anodeof diode D9 is connected to the condenser C9. The cathode of diode D8 isconnected by way of leak resistor R18 to a source of positive potential+12 v. while the anode of diode D9 is connected by a similar leakresistor R19 to a source of negative potential l2 v. The anode of diodeD8 and the cathode of diode D9 are interconnected and joined to oneterminal of a condenser C whose opposite terminal is earthed. Theinterconnected anode of diode D8, cathode of diode D9 and condenser C10are also joined to the phase discriminator terminal 124 In the operationof this phase discriminator circuit as so far described and assumingthat the valve V6 is rendered conductive for a period which isaccurately bisected by the reference edge produced in the mastersquare-wave signal upon the changeover of the trigger circuit of valvesV1 and V 2 from its reset to its triggered condition, then current willflow for an equal period of time in each of the two halves of theprimary winding of transformer T and equal and opposite chargingcurrents will be supplied to the condenser C10, first in positive-goingsense by way of diode D9 and secondly in negative-going sense by way ofdiode D9. The net charge acquired by the condenser C10 is therefore niland no output voltage will be provided at the phase discriminator outputterminal 120. If however the positive transient of the Slave pulsesupplied to the control grid of valve V6 is late, relative to thechangeover instant of valves V1, V2 and the reference edge of the masterwaveform, then current will flow through valve V4 for a longer period oftime than it will flow through valve V5 with the result that thecondenser C18 will receive more current through diode D9 than itreceives through diode D8 and the upper terminal of condenser C10 willacquire a positive charge a negative output control voltage will beproduced at terminal 120. Similarly if the positive transient of theSlave pulse on the grid of valve V6 is early relative to the referenceedge of the master squarewave then more current will be supplied throughthe valve V5 than through the valve V4 with resultant increase ofcurrent through D8 compared with that through diode D9 whereby thecondenser C10 will be charged positively to provide a positive outputvoltage at terminal 120. If the transient of the Slave pulse on thecontrol grid of valve V6 occurs wholly during the time when the triggercircuit of valves V1 and V2 are triggered then all of the current willflow through valve V4 with resultant charging of the condenser C10positively. Con versely if the said transient of the Slave pulse occursduring the period when the trigger circuit of valves V1 and V2 is in itsreset state then all the current will flow through valve V5 withconsequent charging of the con denser negatively.

In the switch valve circuit SVC the valves V7, V8 (conveniently adouble-triode) have their cathodes interconnected and joined by way of aload resistance R22 to a source of negative potential 150 v. The anodeof valve V8 is directly connected to a source of positive potential +200v. while its control grid is connected by way of resistance R21 and lead114 to the output terminal 120 of the phase discriminator PDC. Thegridof valve V7 is connected directly to earth and by way of condenserC11 to the anode of the valve which is joined by way of load resistorR23 to a source of positive potential +300 v. and also to one end of apotentiometer network of resistors R24 and R25 the opposite end of whichlatter is connected to a source of negative potential l50 v. Thejunction between resistors R24, R25 is connected by way of lead to thecathode of the diodes D3 in the master square-wave generator circuitMSG.

In the operation of this switch valve circuit, as the control grid ofvalve V7 is held at earth potential, current will flow in valve V7 andnot in valve V8 whenever a negative voltage is available at the outputterminal 120 of the phase discriminator PDC and under these conditionsthe potential at the anode of valve V7 will be lowered with a consequentlowering of the voltage, relative to earth, of the junction pointbetween resistors R24 and R25. Under these conditions the potential onthe cathode of the diode D3 will be lowered sufiiciently to render itconductive to the p7-Pulse waveform. Such p7-Pulses then operate as aresetting medium for the trigger circuit of valves V1, V2 whereby themaster square- Waveform is reversed back from its positive-going to itsnegative-going state at the time of pulse p7 instead of at the time ofpulse p15 as shown in Fig. 4e. When, however, the output potential onterminal 120 rises above earth, the control grid of valve V8 is likewiseraised whereby this valve becomes conductive and V7 is renderednon-conductive. When this occurs the anode potential of valve V7 movespositively and accordingly raises the potential on lead 110 to bias thecathode of diode D3 positively to an extent whereby it blocks thetransmission of any p7-Pulse therethr-ough. The resetting triggercircuit of valves V1, V2 of the master square-Wave generator MSGtherefore has to await the arrival of the subsequent p15 Pulse asalready described.

Connected to the anode of valve V7 by way of lead 115 and resistor R26are two neon indicator lamps N1 and N2. The opposite electrode of lampN1 is connected by way of feed resistor R27 to a source of positivepotential +200 v. while the other electrode of lamp N2 is connected byway of resistor R28 to a source of positive potential +85 v. CondenserC12 connected between the junction of resistor R26 and the two neonlamps and earth serves as a reservoir condenser whereby the neon lamp N1has sufficient potential impressed thereacross to cause its strikingwhen the anode voltage of valve V7 is lowered i. e. when that valve isconductive, to indicate a Fast running condition and the opposite neonlamp N2 has sufiicient potential impressed thereacross to cause itsillumination when the anode voltage of valve V7 is raised due to thevalve being cut-off thereby indicating a Slow condition of running ofthe mag.- netic drum.

The coarse servo valve circuit CSC comprises valve V3 having its anodedirectly connected by way of lead 109 to the anode of valve V2 of themaster square-wave generator MSG. The cathode of the valve is connecteddirectly to earth while its control grid is joined by way of resistorR30, diode D4, condenser C15 and lead 116 to the anode of valve V6 ofthe phase discriminator 11 PDC. The junction of resistor R30 and diode'D4' (anode) is also connected by way of resistor R311to-a source ofpositive potential+300 'v. and to one terminal of a condenser C14- whoseother terminal is earthed. Diode D5 is connected by its anode to thejunction between the cathode of diode D4 and condenser'ClS and has itscathode earthed.

In the operation of this coarse servo valve circuit, the valve V3 isnormally conducting heavily due to the positive bias supplied to itscontrol grid by way of resistor R31. Its anode current is drawn throughthe anode load resistor R4 of the valve V2 whereby the potential on theinterconnected control grid of valve V1 is held down and-the triggercircuit of valves V1, V2 is locked over in the condition Where theoutput on lead 105 to the phase discriminator PDC iscontinuously at itshigh or positive value and indicative of slow running condition of thedrum MD. Each positive transient of the slave pulses applied to thecontrol grid of valve V6 causes a corresponding negative-going pulse onlead 116 and this is effective via diode D4 to neutralise, in part, thepositive charge on the grid-connected terminal of condenser C14. Onlywhen the frequency of the negative-going output pulses at the anode ofva1veV6 reaches a value sufficiently fully to neutralise the positivebias voltage on the grid of valve V3, is that valve moved towards anodecurrent cut-01f and the trigger circuit of valves V1, V2 freed tooperate as previously described. This is arranged to occur at aboutnormal running speed of the drum.

Referring now to Fig. 6 the output from the phase discriminator PDC onlead 111 is applied to the shaping valve V which is arranged in afeedback type circuit. The input to the control grid of valve V9 is byWay of resistors R33, R34 and R35 of which the resistor R34 is shuntedby condenser C17. Condenser C18 is connected between the junction ofresistors R34, R35 and earth and is shunted by a manual control switch Swhich, when closed, serves to disable the servo control system. Theanode of the valve V9 is connected to a source of positive potential+300 v. by way of load resistor R36 and feedback to the control grid isby way of condenser C19. A further feedback for controlling the responseof the circuit and to limit the degree of overshoot in the servo actionis by way of a series network comprising resistor R37 and condenser C20connected between the control grid of the valve and a tapping point on apotentiometer network of resistors R40, R41 and R42 which is connectedbetween the anode of the valve and a source of negative potential +150v. The tapping point of this feedback path between resistors R40 and R41is also connected to the anode of a diode D10 whose cathode is connectedto a source of positive potential +12 v. and to the cathode of a diodeD11 whose anode is supplied from a source of negative potential +12 v.The cathode of valve V9 in addition to having a cathode load comprisingvariable resistor R39 shunted by fixed resistor R38'for varying thegridto-cathode bias potential and thereby the mean brake current whenthe system is running in synchronism, is also connected by way of aseries network of resistors R43, R44, R45 and R46 to a source ofpositive potential +200 v. The tapping points on this resistance chainare each decoupled to earth by way of condensers C21, C22 and C23 andprovide stabilised output points giving +ll v., +l00 v. and +85 v. foruse in the other parts of the circuit as already indicated. The requiredbiassing voltages of l2 v. and +12 v. already noted are provided from aseparate potentiometer network comprising resistors R53, R54, R55 andR56 connected in series between a source of positive potential +200 v.and a source of negative potential 150 v. The tapping point betweenresistors R54 and R55 is connected directly to earth while the remainingtapping 12 points between resistors R53 and R54 and between resistorsR55 and R56 are decoupled to earth by way of condensers. C24 and C25respectively and provide respectively the supply potentials of +12 v.and -12 v.

The power amplifier circuit PA comprises a pair of parallel-connectedvalves V10 and V11. The input to the control grid of each valve is byway of resistor R47 from the tapping point between resistors R41 and R42on the output network from the shaping valve V9 already described. Thescreen grids of valves V10 and V11 are independently supplied with theiroperating potentials by way of resistors R50 and R51 from the source ofpositive potential +300 v. while both valve anodes are connectedtogether and joined to the source of positive potential +300 v. by wayof resistor R52. The operating coils of the electromagnetic brakingmeans BM are connected across this anode resistor R52 by way of leads113.

The operation of the circuit arrangements described is as follows.Assuming first that the magnetic drum MD is at rest and that current isthen supplied to the driving motor DM; Initially the frequency of theslave pulses derived from the recording track ST, will be low and theresultant negative charging current delivery to the condenser C14 of thecoarse servo valve V3 will be insufficient to neutralise the positivebias-on the control grid of that valve, which therefore remains fullyconducting continuously. This locks the trigger circuit of valves V1, V2in the master square wave generator MSG, in the condition where valve V1is continuously cut-off. In consequence a steady potential equal invalue to that of the positive portions of the master square wave isapplied over lead to the phase discriminator PDC and as a result asteady positive potential is available at the phase discriminator outputterminal and by application to shaping valve V9 provides a steadynegative potential at the control grids of the power amplifier valvesV10 and V11 sufficient to reduce the braking current to zero orsubstantially so. The driving motor DM is thus unhindered in itsacceleration.

These conditions prevail until the drum speed reaches about threequarters that of the proper operating speed whereupon the coarse servovalve V3 becomes cut off by the increased negative bias currentdelivered from the anode of valve V6 to the condenser C14 and thereafterthe trigger circuit of valves V1, V2 becomes fully operative.

In each subsequent beat period of the machine rhythm the trigger circuitof valves V1, V2 will be put into its triggered condition by each 223pulse delivered on lead 106 through diode D1 and as the discriminatoroutput potential is still positive on lead 114, the valve V7 of theswitch valve circuit SVC will still be cut off and a positive biaspotential still maintained on the cathode of diode D3 whereby the nextapplied resetting pulse the p7 pulse on lead 108 is ineffective to resetthe trigger circuit of valves V1 and V2; such resetting to generate theintermediate edge of each cycle of the master square waveform musttherefore await the arrival of the :15 pulse on lead 107. In consequencethe positive-going portion of the master square-wave immediatelyfollowing each reference edge is materially longer than thenegative-going portion which precedes the next subsequent referenceedge, and, as indicated diagrammatically in Figs. 41 and 4g, there willbe, over a plurality of cycles, more slave pulses during positive-goingmaster square wave periods than there are during the negative-goingmaster square wave periods, and as a result the smoothed potential atthe output terminal 120 of the phase discriminator will still bepositive, thereby maintaining the braking current applied to the brakingmeans BM at a relatively low value.

Eventually. when the motor speed is nearly correct several successiveslave pulses will lie in approximate register with therelated referenceedges of the'square 13 waveform as shown in Figs. 4k and 4i whereuponthe smoothed output voltage at the phase discriminator output terminal120 will drop to zero and, after some degree of oscillation or huntingabout the zero point, will settle at a value which adjusts the brakingcurrent delivery from the power amplifier valves V and V11 at thecorrect value to maintain synchronism.

Under these conditions any divergence of the slave pulses from theposition in time where their positive transients are bisected exactly byeach reference edge of the master square-wave, will produce acompensating change in the output potential at terminal 120 thereby toincrease or decrease the braking current as may be required. Thus, ifthe slave pulses tend to creep forwardly, i. e. to be of slightlygreater frequency than the master square-wave, then such pulses willbecome predominantly in phase with the negative portions of the mastersquarewave which precede each reference edge thereby providing anincreased negative output on lead 111 which, acting through the shapingvalve V9, will increase the braking current through valves V10 and V11.Conversely any creep or slip of the slave pulses backwardly, i. e.because the slave pulse frequency is slightly lower than that of themaster square wave, will make such slave pulses predominantly in phasewith the positive portions of the master square wave and thus provide apositive voltage on lead 111 which, through shaping valve V9, reducesthe braking current through valves V10 and V11. It will be observed thatduring such near-synchronised conditions the position of theintermediate or resetting edge y of the master square wave-form isirrelevant.

If, for any reason the speed of the drum increases appreciably abovethat required for correct synchronism whereby successive slave pulsesslip continuously forward with respect to the reference edges of themaster square wave, then over a plurality of cycles from the point wherethe increase of speed commenced, the slave pulses will be predominantlyin phase with the negative-going portions of the master square-wavewhereby the output potential at terminal 120 of the phase discriminatorPDC becomes negative. This applies a bias to valve V8 which causes valveV7 to become conductive and thereby produces a lowering of the voltageat the anode of valve V7 which correspondingly lowers the potential atthe cathode of the diode D3 to an extent whereby the p7 pulses arrivingon lead 108 are rendered effective to reset the trigger circuit ofvalves V1 and V2 ahead, in time, of the previously used p pulses so thatthe master square Wave now has its negative-going portion of appreciablygreater duration than its positive-going portion. Thereafter any furtherincrease of the relative frequency of the slave pulses with reference tothe reference edges of the master square wave will still provide anegative control voltage terminal 120 even although such slave pulsesmay slip past the master square wave at a rate which, with the prior artarrangements using a master square wave of symmetrical form, wouldresult in complete failure of control.

Although the invention has been described particularly with reference tothe synchronising of a magnetic drum store forming part of an electronicdigital computing machine having a cathode ray tube type of main datastore and wherein the slave pulse signals are synchronised with aselected one out of each of a plurality of cyclically repeated trains ofpulses, it will be apparent that the invention is capable of muchbroader application and may be used to synchronise a series of slavepulses with a corresponding series of related single pulses. The use ofother pulses such as (nm)th and (n+m)th pulses for resetting the mastersquare wave generating means, while convenient in the presentapplication where such pulses are available, can be avoided, forinstance, by constructing the trigger circuit of valves V1 and V2 as asingle stable-state trigger circuit having two alternative durations ofits unstable state. One duration could arrange normally to be operativeand the other duration brought into effect under the control of theswitch valve SVC. Similarly, numerous departures may be made even withthe particular arrangements described. For ex ample the portion of themaster waveform which follows each reference edge may be negative-goingrather than positive-going provided that appropriate modifications aremade to the phase discriminator circuit or to the servo sstem, whilesimilarly the servo output may be positive when the slave pulses arepredominantly in phase with the negative half cycles of the masterwaveform, and vice versa.

The values of the circuit components shown in Figs. 5 and 6 are asfollows:

R1 ohms 100,000 R2 do 390,000 R3 do 27,000 R4 do..- 27,000 R5 do 470 R6do 470 R7 do 330,000 R8 do 390,000 R9 do 100,000 R10 do 390,000 R11 do470 R12 do 100,000 R13 do 5,600 R14 do 1,000,000 R15 do 68,000 R16 do100,000 R17 do 100,000 R18 do 120,000 R19 do 120,000 R20 do.. 3,300,000R21 do 470 R22 do 13,000 R23 do 47,000 R24 do 330,000 R25 do 390,000 R26d0 100,000 R27 do 220,000 R28 do 220,000 R30 do 470 R31 do 15,000,000R33 do 470,000 R34 do 2,200,000 R35 do 470 R36 do 82,000 R37 do2,200,000 R38 dn 3,300 R39 do 1,000 R40 do 470,000 R41 do 15,000 R42 do470,000 R43 do 18,000 R44 do 3,300 R45 dn 3,300 R46 do 18,000 R47 do..100,000 R48 do 470 R49 do 470 R50 do 15,000 R51 do 15,000 R52 do 15,000R53 do 39,000 R54 (in 2,200 R55 do 2,200 R56 do 27,000 C1 picofarads 47C2 do 47 C3 do C4 d0 47 C5 do 100 C6 do 470 C7 (in 470 C8 microfarad0.01 C9 do 0.01 C10 do 0.002 C11 picofarads 470 C12 microfarad 0.1 C14do 0.1 C15 picofarads 68 C17 microfarads 0.05 C18 do 0.01 C19 do 0.001C20 do 2.0 C21 do 0.1 C22 do 0.1 C23 do 0.1 C24 do 0.01 C25 d0 0.01

Valves- V1, V2 12AT7 V4, V5 12AT7 V7, V8 12AT7 V3, V9 12AT7 V6 EF91 V10,V11 807 orN78 D1 6D2 D4 6D2 D5 6D2 D6 CG4C D7 CG4C D8 6D2 D9 6332 D11CG4C I claim:

said signal output a master signal of square waveform I having a periodtime equal to the time interval between successive controlling pulsesand said generator also includingsignal-controlled means for renderingthe respective time durations of the positive and negative portions ofeach square Waveform period asymmetrical, a phase I discriminator havinga first signal input for connection to said source of slave pulses, asecond signal inputconnected to said signal output of said generatoranda sig: nal output, said phase discriminator operating to, compare thephases of said derived master square wave signal and said slave pulsesand providing at its signal output an output voltage which is of one orother polarity according to whether said slave pulses are predominantlyin phase with the positive or negative portions,.as the case may be, ofsaid master signal, servo means having a control signal input suppliedwith the signal output of said phase discriminator and an outputconnected to operate said timing control means of said source of slavepulses, said output of said servo means operating to 3djust thefrequency of said slave pulses in a sense dependent upon the polarity ofsaid output voltage of said phase discriminator and frequencydiscriminating means having a signal input also supplied with the outputfrom said phase discriminator and a control signal output connected tooperate said wave-asymmetry control means of said generator when therelative frequencies of said master,

Cir

16 signal and of said slave pulses are unequal to render the timedurations of the positiveand negative-going portions of the mastersquare wave signal asymmetrical in such sense that the mean value ofsaid phase discriminator output over a plurality of slip cycles is ofthe polarity appropriate to correct the frequency of said slave pulses.

2. Apparatus according to claim 1 which includes coarse servo controlmeans having a signal input supplied with a signal derived from saidslave pulses and having an output connected to said master square wavegenerator, said servo control means operating to override the operationof said synchronising means when the absolute frequency of said slavepulses falls below a predetermined value;

3. Apparatus according to claim 1 wherein said slave pulses are derivedfrom the movement of a rotating body and wherein said servo-meansoperate to adjust the speed of rotation of said body.

4. Apparatus according to claim 3 in which said rotating body is drivenby a motor inherently capable of rotating said body at a speed in excessof that required for synchronism of said slave pulses with saidcontrolling pulses and in which said servo means operate to control abraking force applied to said body.

5. Apparatus according to claim 4 wherein said control pulses comprise apredetermined one or nth pulse in each train of a number of cyclicallyrepeated trains of pulses.

6. Apparatus according to claim 5 wherein said square waveform generatoris adapted to be triggered by each of said nth pulses and to be reset toits opposite condition by selected other pulses of said cyclicallyrepeated trains of pulses, the selection of said resetting pulse beingdetermined by the direction of slip.

7. Apparatus according to claim 6 wherein the required asymmetry of saidmaster square waveform is obtained by using either the (nm)th pulse-orthe (n-l-m)th pulse of said cyclically repeated trains of pulses forresetting said trigger circuit.

8. Apparatus as claimed in claim 7 in which said master square wavegenerator comprises an electronic trigger circuit havingtwo-stable-states with two independent controlling inputs, one of saidcontrolling inputs being continuously supplied with said nth pulsesignals and the other controlling input being continuously supplied withthe (nm)th pulses and further supplied by Way of a gate circuit with the(n-l-m)th pulses said gate circuit being controlled by the outputderived from the phase discriminator.

9. Apparatus according to claim 8 wherein said coarse servo controlmeans includes a thermionic valve having at least'a cathode, a controlgrid and an anode having its control grid connected to a capacitancewhich is continuously supplied with current of one polarity by way of aresistance and which is also supplied with current which is of theopposite polarity and is variable in accordanc with the frequency ofsaid slave pulses, said thermionic valve having its anode connectedtothe anode of that one ofthe trigger circuit valves which will ensurethat the trigger circuit is maintained continuously in the conditionwhich provides an output appropriate to increase the drumspeed until thefrequency of the slave pulses exceeds the predetermined value.

10. Electric signal synchronising apparatus for synchronising each pulseof a continuous train of slave pulses derived from the movement of anddependent upon the speed of rotation of a'rotating body having means foradjusting its speed of rotation, with the related pulses of a continuoustrain of controlling pulses which comprises an electric square waveformgenerator triggered by each of said controlling pulses to generate amaster signal of square waveform having a period time equal to the timeinterval between successive controlling pulses, a phase discriminatorconnected to compare the phases of said derived master square wavesignal and said slave pulses 17 v and providing an output which is ofone or other polarity according to whether said slave pulses arepredominantly in phase with the positive or negative periods, as thecase may be, of said master signal, servo means actuated by the outputof said phase discriminator and operating said speed adjusting means ofsaid rotating body to adjust the speed of rotation of said body in asense dependent upon the polarity of said output of said phasediscriminator and frequency-discriminating means actuated by saidphase-discriminator output when the relative frequencies of said mastersquare wave signal and of said slave pulses are unequal for renderingthe time durations of the positive and negative-going portions of themaster square wave signal asymmetrical whereby the mean value of saidoutput over a plurality of slip cycles is of the polarity appropriate tocorrect the frequency of said slave pulses.

11. Apparatus according to claim in which said rotating body is drivenby a motor inherently capable of rotating said body at a speed in excessof that required for synchronism of said slave pulses with saidcontrolling pulses and in which said speed adjusting means comprise abraking device controlled by said servo means.

12. Apparatus for synchronising each pulse of a continuous train ofslave pulses with the related pulses of a continuous train of controlpulses, said slave pulses being derived from and dependent for theirtiming upon the movement of a rotating body which is driven by a motorinherently capable of rotating said body at a speed in excess of thatrequired for synchronism of said slave pulses with said control pulsesand which has variable braking means for altering its speed of rotation,which comprises an electric square waveform generator triggered by eachof said control pulses to generate a master signal of square waveformhaving a period time equal to the time interval between successivecontrol pulses, a phase discriminator connected to compare the phases ofsaid derived master square wave, signal and said slave pulses andproviding an output which is of one or other polarity according towhether said slave pulses are predominantly in phase with the positiveor negative peirods, as the case may be, of said master signal, servomeans actuated by the output of said phase discriminator and operatingto control said variable braking means to adjust the speed of rotationof said rotating body in a sense dependent upon the polarity of saidoutput of said phase discriminator and frequency-discriminating meansactuated by said phase-discriminator output when the relativefrequencies of said master square wave signal and of said slave pulsesare unequal for rendering the time durations of the positive andnegative-going portions of the master square wave signal asymmetricalwhereby the mean value of said output over a plurality of slip cycles isof the polarity appropriate to correct the frequency of said slavepulses.

13. Electric signal synchronising apparatus for synchronising each pulseof a continuous train of slave pulses, whose timing is controllable,with the related pulses of a continuous train of control pulses whichcomprises an electric square waveform generator connected to betriggered by each of said control pulses to generate a master signal ofsquare waveform having a period time equal to the time interval betweensuccessive controlling pulses, a phase discriminator for comparing thephases of said derived master square wave signal and said slave pulsesand for deriving an output which is of one or other polarity accordingto whether said slave pulses are predominantly in phase with thepositive or negative periods, as the case may be, of said master signal,servo means adapted to be actuated by the output of said discriminatorand when so actuated operating to adjust the frequency of said slavepulses in a sense dependent upon the polarity of said output, a firstsource of resetting signals for said generator, said first resettingsignals occurring at instants subsequent to said control pulses whichare more than half said period time later, a second source of resettingsignals for said generator, said second resetting signals occurring atinstants subsequent to said control pulses which are less than half theperiod time later, and switching means actuated by saidphase-discriminator output for controlling which of said first andsecond resetting signals are effective on said generator so that whenthe relative frequencies of said master square wave signal and saidslave pulses are unequal the time durations of the positive andnegative-going portions of the master square wave signal are renderedasymmetrical in such sense that the mean value of said output over aplurality of slip cycles is of the polarity appropriate to correct thefrequency of said slave pulses.

14. Electric signal synchronising apparatus for synchronising each pulseof a continuous train of pulses, whose timing is controllable, with therelated pulses of a continuous train of control pulses, which comprisesan electronic two-stable-state trigger circuit having a triggering inputand a resetting input, a first source of resetting signals for saidtrigger circuit, said first resetting signals occurring at instantssubsequent to said control pulses which are more than half the timeinterval between successive control pulses later, circuit means forapplying said control pulses to said triggering input and said firstresetting signals to said resetting input of said trigger circuit togenerate a master signal of square waveform having a period time equalto the time interval between successive controlling pulses, a phasediscriminator for comparing the phases of said derived master squarewave signal and said slave pulses and for deriving an output which is ofone or other polarity according to whether said slave pulses arepredominantly in phase with the positive or negative periods, as thecase may be, of said master signal, servo means actuated by the outputof said discriminator and operating to adjust the frequency of saidslave pulses in a sense dependent upon the polarity of said output, asecond source of resetting signals for said trigger circuit, said secondresetting signals occurring at instance subsequent to said controlpulses which are less than half the time interval between successivecontrol pulses later, circuit means including controlled switch meansfor applying said second resetting signals to said resetting input ofsaid trigger circuit and frequency discriminating means actuated by saidphase discriminator output and controlling said switch means whereby thetime durations of the positve and negativegoing portions of the mastersquare wave signal are rendered asymmetrical in such a sense that themean value of the output from said phase discriminator over a pluralityof slip cycles is of the polarity appropriate to correct the frequencyof said slave pulses when the relative frequencies of said master signaland said slave pulses are unequal.

15. Electric signal synchronising apparatus according to claim 14 whichincludes coarse servo control means for over-riding the operation ofsaid synchronising means when the absolute frequency of said slavepulses falls below a predetermined value.

16. Electric signal synchronising apparatus according to claim 15 whichincludes frequency sensitive means controlling said switch means tomaintain said switch means open circuit whenever the slave pulsefrequency is below said predetermined value.

17. Electric signal synchronising apparatus according to claim 16wherein said coarse servo control means comprises a thermionic valvehaving at least a cathode, a control grid and an anode, a capacitanceconnected between said control grid and said cathode, a resistorconnected between said control grid and a source of current of onepolarity, a source of current of opposite polarity and variable inaccordance with the frequency of said slave pulses connected to saidcontrol grid and a connection from the anode of said thermionic valve toa point on said two-stable-state trigger circuit which will operate toclamp said trigger circuit continuously in the condition which providesan output appropriate to increase the frequency of said slave pulsesuntil said slave pulse frequency exceeds the predetermined value.

18. Apparatus for synchronising the operation of a magnetic drum typestore in an electronic digital computing machine which comprises anelectric motor for driving said magnetic drum store at a speed which isinherently faster than that required to effect synchronisation,electromagnetic braking means connected to said drum store, said brakingmeans being variable to control the speed of said drum store, arecording of slave pulse signals at predetermined positions in arecording track around said drum store, reproducing means for readingsaid recorded slave pulse signals as electric slave pulse signals assaid drum store rotates, a source of control pulses synchronised withthe signalling rhythm of said computing machine, a master square-wavegenerator controlled by said control pulses to generate a mastersquarewave signal having a period time equal to the time intervalbetween successive control pulses, said square-wave generator includingsignal-controlled means for rendering its square-wave outputasymmetrical in either of two senses of which one has the half cyclefollowing each control pulse longer than the subsequent half cycle andthe other of which has the half cycle following each control pulseshorter than the subsequent half cycle, a phase discriminator suppliedwith the output from said slave pulse reproducing means and said mastersquare-wave signal, said phase discriminator providing an output whichis of one or other polarity according to whether said slave pulses arepredominantly in phase with the positive or negative periods, as thecase may be, of said master square-wave signal, braking current controlmeans controlled by said output from said phase discriminator andsupplying said electromagnetic braking means, and circuit means forapplying the output from said phase discriminator to said signalcontrolled means of said master square-wave generator to control thesense of asymmetry of said generated master square-wave in accordancewith the polarity of the output from said phase discriminator.

19. Apparatus for synchronising the operation of a magnetic drum typestore in an electronic digital computing machine which comprises anelectric motor for driving said magnetic drum store at a speed which isinherently faster than that required to effect synchronisation,electromagnetic braking means connected to said drum store, said brakingmeans being variable to conpulse signals at predetermined positions in arecording track around saiddrum. store; reproducing means for readingsaid recorded slavepulsesignals as electric slave pulse signals as saiddrum store rotates, a source -ofron trol pulses synchronised with thesignalling rhythm of said computing machine, a master square-wavegenerator controlled by said control pulses to generate a mastersquare-wave signal having a period time equal to the time intervalbetween successive control pulses, said squarewave generator includingfirst signal controlled means for rendering its square-wave outputasymmetrical in either of two senses of which one has the half cyclefollowing each control pulse longer than the subsequent half cycle andthe other of which has the half cycle following each control pulseshorter than the Subsequent half cycle, and second signal controlledmeans for suspending its operation so as to provide a continuouspotential output, a phase discriminator supplied with the output fromsaid slave pulse reproducing means and said master squarewave signal,said phase discriminator providing an output which is of one or otherpolarity according to whether said slave pulses are predominantly inphase with the positive or negative periods, as the case may be, of saidmaster square-wave signal, braking current control means controlled bysaid output from said phase discriminator and supplying saidelectromagnetic braking means, circuit means for applying the outputfrom said phase discriminator to said signal controlled means of saidmaster square-wave generator to control the sense of asymmetry of saidgenerated master square-wave in accordance with the polarity of theoutput from said phase discriminator, a frequency-sensitive circuitsupplied with said slave pulse signals and providing an output signal ofpredetermined polarity whenever the frequency of said slave pulsesignals is below a chosen absolute valueand circuit means for applyingsaid output from said frequency-sensitive circuit to said second signalcontrolled means of said square-wave generator.

References Cited in the file of this patent UNITED STATES PATENTS2,406,978 Wendt et al. Sept. 3, 1946 2,495,946 Schuler Jan. 31, 19502,558,358 Hales June 26, 1951 2,614,169 Cohen et al. Oct. 14, 19522,652,554 William et al Sept. 15, 1953

